Generally, the digital images displayed by a screen, for example, a television screen, are first transmitted in compressed form to a decoder connected to the screen, for example, in compression formats such as HEVC, H.264, MPEG, JPEG, etc., which provide high compression rates but require relatively complex calculations for their decoding. The compressed images are decoded by the decoder, which generates digital images to be displayed on the screen. An image to be displayed generated by the decoder corresponds to a pixel array, each pixel including a plurality of values or samples corresponding to the different color components of the image, for example, red, green, and blue components R G, and B or, as a variation, luminosity and chrominance components Y, Cr (corresponding to the difference between blue component B and luminance Y) and Cb (corresponding to the difference between red component R and luminance Y). Such an image, generally called “raster” image, corresponds, pixel by pixel, to the image which should be displayed on the screen. This image is first stored in a video memory of the screen, and then read from this memory, for example, line by line, to be displayed on the screen.
In practice, due to the high resolutions (number of pixels per image) and frequencies (number of images displayed per second) of current screens, the flows of images to be displayed generated by the decoders correspond to highly significant data. This may raise issues, particularly on transmission of the images to be displayed from the decoder to the screen, typically over a wire link, for example, an HDMI link, or during phases of temporary storage of the decoded images in an internal memory of the decoder. In particular, the transfer speeds necessary to transmit the decoded images are particularly high, which raises issues in terms of cost, power consumption, and/or generation of electromagnetic disturbances.
For this reason, the digital images to be displayed generated by the decoder are generally compressed by the latter before being transmitted to the screen.
To perform such a compression, it has especially been provided to use lossless compression methods. Such methods however have relatively low compression rates, and are not always sufficient to meet the encountered constraints.
It has further been provided to use so-called visually lossless compression methods, that is, compression methods generating a loss of information, but where the coding errors introduced are selected to be imperceptible or tolerable for the human eye. Examples of such compression methods are for example described in U.S. Pat. No. 6,108,381 (incorporated by reference), and in the article entitled “A novel adaptive quantization method for memory reduction in MPEG-2 HDTV decoders” by R. Bruni et al. (incorporated by reference).
There is a need for a compression method complementary or alternative to existing methods, which enables to decrease the bandwidth required to transmit images to be displayed, without significantly altering the visual quality of the image.